Workpiece carrier

ABSTRACT

A workpiece carrier has a support bar centrally supported at one end of a stem portion. The stem is carried at its opposite end on a base. Columns are interposed between the ends of the support bar and the base. Heat is applied to the columns, preferably by resistive heating elements, to heat them and cause their thermal expansion, causing the support bar ends to be bent away from the base. A relatively flexible workpiece which has been bonded to the support arms on a surface facing away from the base will undergo the same bend.

BACKGROUND OF THE INVENTION

In certain machining operations where the cutting is performed by gritor teeth carried on a surface of highly precise flatness, it isdesirable to regulate the final profile of the machined surface, so asto locate all of a series of features carried on a surface intersectingthe one machined to within a preselected tolerance band from themachined surface. In one such typical operation, 13 features are locatedon 0.16 in. centers and ideally the surface must be machined so thateach feature is finally located from 20 to 80 μin. from the surface. Itis desirable to produce this final configuration regardless of thespacing of these features from either the original surface to be lappedor otherwise machined or from another datum line.

The typical operation mentioned above arises in the batch fabrication ofthin film magnetic heads for disk memories. The features compriseindividual thin film head throats deposited on the workpiece's side andso located, relative to the original position of the surface to bemachined, as to require substantial machining of the surface. After themachining process is complete, the bar is sliced transversely to freethe individual transducer heads.

These heads can not be deposited precisely along any datum line due toerrors in the mask controlling the deposition, errors in position of themask, and changes in bar geometry caused by external force and residualstress. Nonetheless, all of the head throats must be located within thepredetermined 20-80 μin. tolerance band from the edge in order for themto be acceptable. Obviously, the fewer reject heads resulting from agiven workpiece, the more efficient and economical the operation is.Heretofore it has not been possible to locate high percentages of thefeatures simultaneously within the specified tolerance band at any stageof the machining process.

It is also possible to place sensors at predetermined positions on thebar to be machined, to determine the progress of the machiningoperation. In fact, the sensors which we are using provide a continuousand highly accurate indication (to 5 μin. or better) of the spacing ofthe surface being machined from each feature. Therefore, it is possibleto monitor during the machining process the progress of each of thefeatures toward the preselected tolerance band.

PRIOR ART STATEMENT

The best art of which we are aware is the IBM Technical DisclosureBulletin (TDB). Vol. 24, No. 1A, p. 198, June 1981, which discloses foruse in lapping thin film heads, a workpiece support which provides"automated compensation for bow errors". IBM TDB, Vol. 23, No. 4,September 1980; Vol. 13, No. 4, September 1970 and U.S. Pat. No.3,821,815 disclose art pertaining to the lapping or machining steps inbatch fabricating thin film transducer heads.

BRIEF DESCRIPTION OF THE INVENTION

Our invention is a workpiece carrier tool useful for increasing thepercentage of features which fall within the tolerance band. Toaccomplish this, we propose to bend the workpiece during machining sothat all or at least most of the transducers fall within a toleranceband of the desired width. The way we accomplish this is by bonding theworkpiece to the carrier tool. In our preferred embodiment, the carrierincludes a support bar supported at its center in spaced apartrelationship from a relatively rigid base by a stem, so that each end ofthe support bar is cantilevered in effect, from the stem. A pair ofcolumns supported by the base on opposite side of the stem extend towardand contact load points on the ends of the support bar. These columnshave positive thermal coefficients of expansion.

The support bar and the workpiece bonded to it can be bent by applyingheat to each column, causing the column to expand and apply force to thesupport bar. We prefer to apply the heat with a resistance elementthrough which current can be passed in an axial bore within each column.As much as 400 μin. adjustment may be needed in the relative position ofthe features carried on the workpiece in the previously mentionedtypical operation. For a typical metal which has a coefficent of thermalexpansion of around 10⁻⁵ in./in./°F., one can see that the necessarybending of the support bar can be accomplished with a temperature riseof only a few tens of degrees Fahrenheit, if each column is an inch orso long, has a sufficiently large solid area cross section, and thesupport bar is not excessively stiff.

By monitoring the distance between each transducer and the toleranceband, one can easily determine the bending of the support bar necessaryto accomplish the goal of increasing the amount of machining whichoccurs at those transducer stations whose features are still relativelyfar from the tolerance range.

Accordingly, one feature of this invention is to provide a means forsupporting a workpiece during machining operations.

Another purpose is to provide a means for bending the workpiece whilemounted on the support.

Still another purpose is to control such bending so as to allow a seriesof partially aligned features on the workpiece to all enter apreselected tolerance band before any have moved out of said toleranceband during the machining operation.

Another purpose is to reduce the scrap rate for the articles beingsimultaneously produced by the machining of the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a preferred workpiece to be supported by this carrier tool.

FIG. 2 is a graph showing the distances of individual features carriedon a side of the workpiece, from an edge of the workpiece being shiftedby the machining operation.

FIG. 3 is a graph showing the same distances as in FIG. 2, shifted bythe machining of approximately 30 μin. from the workpiece edge.

FIG. 4 is a graph showing the results of using this carrier to bendduring the machining operation a workpiece with the graphiccharacteristics of FIG. 2 and thereby change the relative positions ofthe features from the machined edge.

FIG. 5a shows a preferred embodiment of the invention, with theworkpiece mounted on it.

FIG. 5b is a cross section view of the insulating spacers 27 separatingcolumns 17a and 17b from set screws 26.

FIGS. 6a and 6b show in exaggerated form two appearances of theworkpiece carrier during a preferred method of attaching the workpieceto it.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The section of a typical workpiece 11 shown in FIG. 1 has an arrangementof symbolically displayed features 15 which may be the throats of thinfilm magnetic transducers for use in disk memories between which arelocated so-called machining guides or sensors 14. As a practical matter,since it is essential for the successful operation of the invention thatfeatures 15 and the sensors 14 immediately adjacent them be preciselyspaced respective each other relative to line 29, reference to a feature15 and a sensor 14 adjacent it are equivalent. That is, when a sensor 14is known to have a certain spacing from line 29 with very little error,one then knows the spacing between an adjacent feature 15 and line 29with very nearly the same accuracy. Forming such an accurate positionalrelationship between features 15 and sensors 14 can be done according tothe teachings of copending patent application Ser. No. 430,194, entitledMethod of Calibrating a Machining Sensor, having Holmstrand, Hennefentand Kracke as coinventors, and having a common assignee with thisapplication. In the typical operation discussed supra, workpiece 11 has14 machining sensors 14, which are numbered from left to right, 1through 14 as shown in FIG. 5.

In the typical operation, the face 33 of workpiece 11 is slowly lappedby a lapping wheel causing edge 29 to shift slowly toward and throughsensors 14. Although the critical feature, throat height, of eachtransducer 15 is quite accurately placed on workpiece 11 relative toadjacent sensors 14, the relatively great length of workpiece 11 and themanufacturing tolerances involved means it is not possible that everysensor 14 or feature 15 will be located at identical spacings fromeither the initial position of line 29 or intermediate positions as line29 is shifted towards guides 14 and transducers 15 during the machiningoperation.

To more clearly illustrate this situation, the graphs of FIGS. 2-4 showthe distances of individual sensors 14 (or features 15) from line 29 ona greatly expanded scale for various situations in the typical machiningoperation. In each of FIGS. 2-4, the small dashes indicate on theordinate scale the spacing of the sensor 14 (or feature 15 adjacent it)whose position number appears on the abcissa, from edge 29. For thetypical application here, there is a head throat height tolerance rangefor the transducers of from 20 to 80 μin. corresponding to the twohorizontal lines at these points in FIGS. 2-4.

Thus, in FIG. 2 edge 29 has due to the initial machining approached thevarious sensors 14 from a typical initial spacing of several hundredmicroinches. At this stage for the particular workpiece 11 involved,sensors 14 having position numbers 4-8 have fallen within the tolerancerange and sensors 14 having positions 1-3 and 9-14 have not yet movedwithin the tolerance range. In FIG. 3, about 30 μin. more has beenground from face 33, and the same sensors have been positioned nearer toedge 29, so that sensor 14 at position 5 is nearly at the low end of thetolerance range and sensors at positions 12-14 are still above themaximum tolerance level of 80 μin. spacing from edge 29.

It can be seen that sensors 14 located at positions 4-8 will fall belowthe tolerance range if machining continues until sensors 14 at positions1, 2 and 12-14 fall within the tolerance range. Therefore, the mostfavorable point to stop machining is that shown in FIG. 3 since 9 out ofthe 14 sensors involved will have the proper tolerance relative to edge29, and therefore the largest number of acceptable transducers 15 willbe produced. It is also clear, that if some means of causing moremachining to occur in the vicinity of sensors 14 numbered 1, 2 and 12-14was available, that all of the transducers 15 carried on workpiece 11could be made useable.

The workpiece carrier 10 shown in FIG. 5 is one means of providing thiscapability. A relatively rigid base 23 has integral with it at itsmidpoint one end of a stem portion 21 which projects perpendicularlyfrom base 23. Support bar 12 is fixed approximately midway along itslength to, and preferably is integral with, the other end of stem 21,and extends approximately parallel to base 23. Each end of bar 12 isthus cantilevered from stem 21 and is held in opposing, spaced apartrelationship to base 23. Workpiece 11 is bonded by adhesive in adhesiveline 31 to faces 32. Columns 17a and 17b are positioned in bores 19.Bores 19 pass entirely through base 23, and are located so that theiraxes intersect load point areas near each end of support bar 12. Aportion of each bore 19 adjacent the end further from bar 12 is threadedso as to mesh with a set screw 26. Each set screw 26 has an internalconcentric bore.

When bar 12, stem 21 and base 23 are chosen from a thermally conductiveand thermally expanding material, it is necessary to thermally insulatecolumns 17a and 17b so as to prevent heat transfer to the remainder ofthe carrier 10. Accordingly, an end plug 16 made of some low heatconductivity material is mounted at the end of each column 17a and 17badjacent their load point areas. The plugs 16 preferably have adome-shaped head and a shaft projecting from it which fits snugly into abore 22 of the column 17a or 17b to keep plugs 16 precisely interposedbetween the load point area and the adjacent column 17a or 17b. Acylindrical spacer 27 with an inside diameter slightly greater than theoutside diameter of columns 17a and 17b fits within each bore 19 atopthe set screw 26 engaging the threads in bore 19. An inner shoulder 40(FIG. 5b) partially closes the ends of spacers 19 adjacent set screws26, thereby spacing the end of column 17b from its adjacent set screw26. Spacers 27 can be made from the same material that plugs 16 aremade. The material for these plugs 16 and caps 27 must have substantialmechanical compressive strength.

Each column 17a and 17b as mentioned above, has a bore 22 extendingalong substantially all its length, and which communicates with the boreof the adjacent set screw 26 through the end of spacer 27. Heaterelements 24a and 24b are located respectively within the bores 22 ofcolumns 17a and 17b, and have respectively a pair of wires 25a and 25bfor conducting electric current to the associated element 24a and 24b.Elements 24a and 24b are in intimate thermal contact with columns 17aand 17b respectively. Wires 25a and 25b pass through the bores of setscrews 26, and thus are available to be connected to a convenient powersource. The bores 42 and 41 respectively in set screws 26 and (FIG. 5b)spacers 27 are preferably large enough to allow a heater element 24a and24b to be inserted into or withdrawn from column 17a or 17brespectively.

Workpiece 11 is attached preferably by gluing to the support face 32.Grooves 13 permit cutting the workpiece into individual transducerassemblies after face 33 of workpiece 11 has been machined to the properposition relative the sensors 14. The entire carrier can be mounted byclamping it on a tool arm for support relative to the machine cutter orabrasive-covered disc.

There are several considerations which are necessary so that thisarticle will function properly. It is necessary that bar 12 berelatively flexible in comparison to base 23. This is accomplished inour design simply by making the thickness (vertical height in FIG. 5a)relatively smaller for support bar 12 than for base 23.

Columns 17a and 17b must be selected from materials which have asubstantial positive thermal coefficient of linear expansion and whichalso have a relatively high modulus of elasticity. Ferrous alloys(particularly stainless steel), certain aluminum alloys, and brass allhave relatively high thermal expansion coefficients, on the order of6-10 μin./in.°F. It is also useful that columns 17a and 17b haverelatively high thermal conductivities so that the heat from elements24a and 24b is rapidly distributed through the entire masses of columns17a and 17b. We prefer stainless steel for columns 17a and 17b. A column17a or 17b which is one inch long and made of stainless steel willexpand approximately 10 μin. for each °F. increase it experiences. A 10°F. temperature change in columns 17a or 17b thus causes a 100 μin.expansion of such a column when its ends are not restrained. So as tointroduce no errors caused by differing rates of thermal expansion, weprefer that bar 12, stem 21, and base 23 all be made from the materialconstituting columns 17a and 17b. It appears that bending can becontrolled to within a few microinches.

In operation, electric current is applied to one or both of the pairs ofconductors 25a and 25b, causing the heater element 24 connected theretoto produce heat, warming the column 17a or 17b enclosing it. This warmthcauses the previously explained expansion of the column involved,producing force against the arm of support bar 12 at the associated loadpoint. This force causes support bar 12 to bend, giving workpiece 11 aslightly concave shape. The amount of bend is dependent on the crosssectional area and the elastic modulus of columns 17a and 17b, and thestiffness of bar 12 and base 23, in accordance with well-knownprinciples of structural analysis. Therefore, one must determine theamount of deflection needed in bar 12, and then design columns 17a and17b and bar 12 to provide enough force for the temperature rangeachievable to cause the deflection needed. Control of current flowthrough heating elements 24 permits regulation of the temperature ofcolumns 17a and 17b with reasonable accuracy, and thereby control of thedeflection of support bar 12. Referring to FIGS. 3 and 4, one can seethat bending workpiece 11 concavely by about 40 μin. will cause the endsto be machined more than the center, and sensors 14 at stations 1, 2 and12-14 to be brought within the tolerance range after more machining.

It is necessary that set screws 26 be tightened sufficiently to givepreload to columns 17a and 17b. The amount of preload depends on thepreviously mentioned mechanical characteristics, and can be accuratelyset by proper torquing of set screws 26 once the configuration of arm12, columns 17a and 17b, and base 23 has been finally selected.

It may also be desirable to provide an initial slight convex prebend toworkpiece 11 prior to any bending of support bar 12. This is preferablyaccomplished by heating columns 17a and 17b before workpiece 11 isbonded to bar 12. Workpiece 11 is then bonded in place as shown in FIG.6a. When columns 17a and 17b are then cooled, they contract and bar 12returns to its undistorted shape as shown in FIG. 6b. Workpiece 11 isconstrained by the adhesive to take the bend caused by bar 12'sstraightening.

To insure accuracy and repeatability of the bonding of workpiece 11 tobar 12, workpiece 11 must initially have little residual stress in itwhile the bonding occurs. Accordingly, we have developed a preferredmethod for accomplishing the bonding. This method involves first layingworkpiece 11 directly on upwardly facing faces 32. Then a hypodermicneedle 34 or other applicator is used to place drops 35 of a liquidadhesive having low viscosity and surface tension at regular intervalson faces 32 adjacent a side of workpiece 11. This adhesive must be ofthe type which hardens shortly after contact by air, and which can bedissolved in some manner when one desires to detach a workpiece 11 fromsupport 10. Capillary action draws the adhesive into the cracks betweenfaces 32 and workpiece 11. The adhesive then bonds workpiece 11 to faces32 when it hardens forming adhesive lines 31. Workpiece 11 is detachedby soaking the entire assembly in an appropriate solvent. The adhesivematerial which we currently favor is the cyano-acrylate known asSuperbonder 916 temporary bonding adhesive, which is manufactured byLoctite Corp., Newington, CT 06111. This material has a viscosity of 1-5centipoise at 25° C. and a suitable surface tension. The cured adhesiveis dissolved by nitromethane or acetone. The surface tension andviscosity values must be respectively large and small enough to causethe adhesive to be drawn into the gap between the faces 32 and theworkpiece 11 by capillary action. By using this non-pressure techniqueto bond the workpiece 11 to bar 12, little or no initial stress ispresent in the workpiece 11. Furthermore, the use of this techniqueavoids the necessity of accurate machining of the surface of workpiece11 bonded to faces 32. Irregularities in the gap are simply filled withadhesive. When applying the adhesive, it is desirable that the ends ofworkpiece 11 are bonded to bar 12 first. Once the ends have been fixed,then we prefer to alternately proceed from face 32 to face 32 inwardlywith the bonding.

Turning now to miscellaneous variations of this invention, it ispossible to supply the heat to columns 17a and 17b by means other thanelectrical heating elements 24 and 24b but no other method appears to beas efficient or easily controlled as is electricity.

Another variation possible is to cantilever bar 12 at one end from stem21 and employ only a single column 17a. Thus only one end of bar 12 isbent when current is applied to heat column 17a. At this time, thisdesign does not appear to be as desirable as the preferred embodimentdescribed above, because it is not possible to bend the two ends of bar11 different amounts.

Again, if bar 12, stem 21, and base 23 are made from a refractory orother material which has low thermal conductivity or a small coeffecientof thermal expansion, then it would not be necessary to thermallyinsulate columns 17a and 17b from base 23 or bar 12.

What is claimed is:
 1. An article for supporting an elongate workpiecehaving first and second elongate, flat, substantially parallel surfaces,said workpiece to be attached to the article by bonding the firstsurface of the workpiece to a support surface on the article and forbending the workpiece to create a concave shape in its second surfaceresponsive to power input comprising;(a) a relatively rigid base; (b) astem having first and second ends and attached at its first endapproximately perpendicular to the base; (c) a support bar attached tothe second end of the stem and cantilevered therefrom approximatelyparallel to and opposing the base, said support bar being relativelyflexible respective to the base's and stem's stiffness, and said supportbar having a support surface facing away from the base and conformingapproximately to the shape of the workpiece's first surface, and havinga load point facing the base on the cantilevered end of the support bar;(d) a column attached to the base and spaced apart from the stem, andextending toward and contacting the load point on the support bar, eachsaid column having a positive thermal coefficient of expansion; (e) heatgenerating means in intimate thermal contact with the column, andreceiving the energy input and generating heat therefrom, for heatingthe column and causing it to expand, thereby applying force to the loadpoints and bending the support bar.
 2. The article of claim 1, whereinthe stem is attached to the base at approximately the base's midpoint;wherein the support bar is attached to the stem at approximately thesupport bar's midpoint and has a load point at each end facing the base;and further comprising a pair of columns supported by the base onopposite sides of the stem and extending toward and contacting the loadpoint thereon.
 3. The article of claim 2, wherein the base, stem andsupport arm are an integral piece of metal.
 4. The article of claim 2,wherein the energy input is electric current, and the heat generatingmeans comprise a resistive element and means for conducting the currentthrough the resistive element.
 5. The article of claim 4, wherein eachcolumn has a longitudinal bore containing the resistive element.
 6. Thearticle of claim 4, including thermal insulating means located at eachend of each column for resisting flow of heat from the columns to thesupport arm end base.
 7. The article of claim 4, wherein the baseincludes a set screw means mechanically supporting the column endsupported by the base for preloading each column.
 8. The article ofclaim 7, wherein the set screw means comprises areas in the basecontaining threaded holes whose axes intercept the load points, and aset screw in each hole contacting an end of one column.
 9. The articleof claim 8, wherein the set screw means further comprises a thermalinsulating bushing contacting and surrounding the column adjacent thebase, wherein the set screw and the bushing contain concentric axialbores through them, and wherein each column contains an axial borecommunicating with the bore in the adjacent set screw.
 10. The apparatusof claim 1, wherein the stem and the columns comprise materials havingsimilar coefficients of thermal expansion.
 11. The apparatus of claim 1,wherein the support surface includes areas containing transverse slotshaving predetermined spacing.